M. A. Nelson

Double β decays of100Moand150Nd

The double beta decays of Mo-100 and Nd-150 were studied in a time projection chamber located 72 m underground. A 3275 h exposure of a 16.7 g sample of metallic Mo enriched to 97.4 % in Mo-100 resulted in a two-neutrino half-life of (6.82 + 0.38 - 0.53 +/- 0.68) * 10**18 y. Similarly, a 6287 h exposure of 15.5 g of Nd2O3 enriched to 91 % in Nd-150 yielded (6.75 + 0.37 - 0.42 +/- 0.68) * 10**18 y. Lower limits on half-lives for neutrinoless decay with and without majoron emission also have been measured.

Double Beta Decay of 48 Ca

{sup 48}Ca, the lightest experimentally accessible double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus the {beta}{beta}{sub 2{nu}} half-life measurement, reported here, provides a unique test of the nuclear physics involved in the {beta}{beta} matrix element calculation. Enriched {sup 48}Ca sources of two different thicknesses have been exposed in a time projection chamber. We observe a half-life of T{sub 1/2}{sup 2{nu}}=(4.3{sub {minus}1.1}{sup +2.4}[stat]{plus_minus}1.4[syst]){times}10{sup 19} yr, consistent with shell model calculations. {copyright} {ital 1996 The American Physical Society.}

Tracking electrons from double beta decay — How far can you push the TPC?

Abstract New results are reported from time-projection-chamber measurements of the double beta decay of 100Mo and 150Nd. A previously-observed high-energy anomaly has been eliminated by improved energy resolution. Kurie plots of the two-neutrino spectra show end-point energies close to the reported parent-daughter mass differences. The 150Nd source has produced a new direct-counting 90% confidence neutrino-majoron coupling limit of 〈gν,χ〉

Double Beta Decay of 48Ca

{sup 48}Ca, the lightest experimentally accessible double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus the {beta}{beta}{sub 2{nu}} half-life measurement, reported here, provides a unique test of the nuclear physics involved in the {beta}{beta} matrix element calculation. Enriched {sup 48}Ca sources of two different thicknesses have been exposed in a time projection chamber. We observe a half-life of T{sub 1/2}{sup 2{nu}}=(4.3{sub {minus}1.1}{sup +2.4}[stat]{plus_minus}1.4[syst]){times}10{sup 19} yr, consistent with shell model calculations. {copyright} {ital 1996 The American Physical Society.}

Observed double beta decay spectra from 82Se, 100Mo, and 150Nd☆

Abstract Two-electron events resembling double beta decay are being observed at energies beyond the die-off of the spectrum predicted for the two-neutrino mode. The anomaly appears in three isotopes having different half lives and Q-values. Tests are now underway to determine its origin.

A measurement of the double beta decay half-life of 48Ca

48 Ca is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a ββ 2 v measurement of this isotope provides a unique test of the nuclear physics involved in ββ matrix element calculations. Enriched 48 Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T 1/2 2 v = (5.5 −1.5 +3.5 ) × 10 19 y, in agreement with shell model calculations.

The double beta decay spectra of 82Se, 100Mo, and 150Nd

The double beta decay electron energy spectra of 82Se, 100Mo, and 150Nd have been measured with a time projection chamber, and departures from the expected two‐neutrino spectral shapes have been observed. Efforts to reduce possible background contamination have been made, and tests are now being done in an effort to determine whether the anomalous signals are real effects, or simply experimental artifacts.

Double beta decay of48Ca

Abstract48Ca, the lightest double beta decay candidate, has been until now the only one simple enough to be treated exactly in the nuclear shell model. Thus, theββ(2ν) half-life measurement, reported here, provides a unique test of the nuclear physics involved in theββ matrix element calculation. Enriched48Ca sources of two different thicknesses have been exposed in a time projection chamber, and yieldT12/2ν=(4.3−1.1+2.4[stat.] ± 1.4[syst.]) × 1019 years, compatible with the shell model calculations. The consequences of this result for the shell model calculation of theββ(2ν) rate are briefly discussed.

Double beta decay ofSe82

Pairs of negative beta particles have been observed originating from a /sup 82/Se source during a cloud-chamber search for double beta decay. Backgrounds recognized in previous experiments were suppressed to well below the observed event rate, and no other significant backgrounds are apparent. Within the limited statistics of the small data sample, the observed single-electron energy spectrum, the two-electron sum energy spectrum, and the opening angle distribution are consistent with expectation for neutrino-accompanied double beta decay of /sup 82/Se. The tentative assignment of the observed events to double beta decay, results in a /sup 82/Se half-life of (1.0 +- 0.4) x 10/sup 19/ years, in good agreement with some very recent theoretical predictions. However, the result is in serious disagreement with the much longer half-lives measured in geochemical experiments. A planned follow-up experiment is described.